This invention relates generally to signal translation systems for integrated circuits and specifically to enhanced PNP turnaround circuits therefor using common base connected lateral PNP transistors. The invention also broadly relates to common base connected lateral PNP transistors in integrated circuits for level shifting DC coupled signals. As is known, a lateral PNP is an integrated circuit transistor with the collector available for external connection.
Integrated circuit design has a number of recurrent problems, including the practical requirement for DC coupling between successive transistor stages, which limits the number of successive gain stages. This is due to the inability to fabricate suitably sized integrated circuit capacitors for low frequency signals, such as television video signals, for example. There is also a problem in obtaining single-ended push/pull output signals with good frequency response. This is due to the need to use PNP type transistors in common emitter configurations.
In integrated circuits, the differential mode of connection of transistors is used extensively because of the inherent similarity of the transistors produced during manufacture, which yields a very high degree of stability and predictability in the matched pairs.
It is well known that transistors operated in the common emitter mode exhibit good power gain, whereas those operated in the common base mode exhibit good voltage gain. It is also well known that a typical common emitter NPN transistor has a frequency response substantially greater than its common emitter PNP counterpart. For example, a typical NPN--3dB point is around 5 MHz whereas for a PNP it is around 300 KHZ. However, the usable frequency range for a common base connected PNP transistor is close to that of a common emitter connected NPN transistor.
The difficulty in obtaining push/pull outputs in integrated circuits is that the PNP frequency response is so poor in the common emitter configuration. It is known to use a common base lateral PNP connected in a so-called turnaround circuit having a PNP current mirror in one side of a differential amplifier to interface with the NPN stages. Unfortunately, the PNP current mirror is connected in common emitter fashion and limits the frequency response of the circuit.
The need to DC couple successive NPN stages results in the latter ones of the cascaded stages being operated at successively higher DC voltages. The operating voltage constraints on the integrated circuit chip impose practical limits, unless PNP stages or AC coupling is used. In television video circuits, where the video information includes 60 Hz vertical synchronizing signals as well as signals in the MHz range, external capacitors are needed. Thus, not only are extra "pin-outs" required on the IC but labor cost, size and reliability are all affected. Clearly, there is a need in integrated circuits for a PNP transistor arrangement for DC level shifting without sacrifice of frequency response.